Dc resistance welding apparatus

ABSTRACT

A DC resistance welding apparatus includes a pair of welding electrodes for sandwiching workpieces to be welded therebetween and a circuit connected to the welding electrodes for supplying a welding current to said welding electrodes. The circuit comprises includes a plurality of parallel-connected rectifying circuits each comprising having a welding transformer and rectifying circuit. The circuit allows a large welding current to be supplied to workpieces, and makes the DC resistance welding apparatus small in size and light in weight.

BACKGROUND OF THE INVENTION

The present invention relates to a DC resistance welding apparatus forresistance-welding workpieces with a direct current, and moreparticularly to an inverter-type DC resistance welding apparatus forrectifying three-phase AC electric energy into DC electric energy,converting the DC electric energy to high-frequency AC electric energy,then converting the high-frequency AC electric energy again into DCelectric energy with an welding transformer and rectifiers, and thensupplying the DC electric energy to welding electrodes to weldworkpieces.

Resistance welding apparatus include a pair of electrodes for gripping aset of workpiece therebetween. While a welding current is being suppliedbetween the electrodes to generate Joule heat, the electrodes arepressed against the workpieces to weld the workpieces to each other. Theresistance welding is highly efficient as it requires no welding rods.

The resistance welding process requires an electric current much greaterthan other welding processes such as the arc welding process. Therefore,the welding transformer used by the resistance welding process is largeand heavy. The large and heavy welding transformer is one of thedrawbacks which make it difficult to install the resistance weldingapparatus on the arm of a welding robot or the like.

To make the welding transformer smaller in size, there has recently beenemployed an inverter-type DC resistance welding apparatus which convertsDC electric energy to high-frequency AC electric energy, supplies thehigh-frequency AC electric energy to a welding transformer to lower thevoltage thereof, then converts the high-frequency AC electric energy toDC electric energy, and supplies the DC electric energy to welding gunarms. The DC electric energy is first converted to the high-frequency ACelectric energy because the high-frequency AC electric energy allows thewelding transformer to be relatively small and light-weight since thecross-sectional area of the core of the welding transformer is inverselyproportional to the frequency of the high-frequency AC electric energy.The reason for converting the high-frequency AC electric energy back tothe DC electric energy for application to the welding gun arms is thatthe DC electric energy supplied to the welding gun arms avoids a voltagedrop which would otherwise be caused by an increased high-frequencyimpedance due to the stray inductance resulting from the length andshape of the welding gun arms, and also avoids a voltage drop whichwould otherwise be developed by the skin effect of the welding gun arms,so that the welding apparatus is highly efficient in operation.

One inverter-type DC resistance welding apparatus is shown in FIG. 1 ofthe accompanying drawings. The DC resistance welding apparatus comprisesa converter unit 2, an inverter unit 4, and a welding transformerassembly 6 including a welding transformer 14. Three-phase AC electricenergy from a commercial three-phase AC power supply 7 is converted toDC electric energy by a rectifier 8 and a capacitor 10 of the converterunit 2, and the DC electric energy is then converted to AC electricenergy having a frequency higher than that of the three-phase ACelectric energy by the inverter unit 4 which comprises a bridge oftransistors 12a through 12d. The high-frequency AC electric energy isthen converted to DC electric energy again by the welding transformer 14with a central tap 19 on its secondary winding and rectifiers 16a, 16b.The DC electric energy is then supplied between welding electrodes 18a,18b.

The welding electrode 18a is connected to the common joint between therectifiers 16a, 16b, and the welding electrode 18b is connected to thecentral tap 19. A pair of workpieces Wa, Wb to be welded together isplaced between the welding electrodes 18a, 18b. When a welding currentis passed between the welding electrodes 18a, 18b through the workpiecesWa, Wb, the contacting surfaces of the workpieces Wa, Wb are melted andwelded to each other.

DC resistance welding apparatus are always required to supply a largewelding current and to be small in size. A large welding current ispreferable when welding steel sheets such as plated steel sheetscontaining materials of different melting points or welding aluminumsheets or the like having a large thermal conductivity.

If the current capacity of the conventional DC resistance weldingapparatus is to be increased and the DC resistance welding apparatus isto be installed on the arm of a welding robot, then the weldingtransformer 14 will be increased in weight, requiring the robot to belarge in size, and it is required to connect the transistors 12a through12d of the inverter unit 4 or the rectifiers 16a, 16b parallel to eachother. As a result, the DC resistance welding apparatus may havestability and reliability problems.

Heretofore, the transistors of the inverter are operated within acontinuous DC rating range. The inverter is designed such that even ifdriver circuits connected to the bases of the transistors malfunctionand the transistors remain conductive, the inverter will not be brokeninsofar as it is within its thermal limits. The actual operating rangeof the inverter is therefore required to be smaller than the continuousDC rating range. As a consequence, the conventional DC resistancewelding apparatus is not suitable for use in applications in which alarge welding current is to be supplied to the welding electrodes.

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide a DCresistance welding apparatus which has a plurality of parallel-connectedrectifying circuits each comprising a welding transformer andrectifiers, so that a large welding current can be supplied toworkpieces, and which is small in size and light in weight.

Another object of the present invention is to provide a DC resistancewelding apparatus having a means for monitoring the time in whichsemiconductor devices such as transistors of an inverter unit areconducted, to allow the semiconductor devices to be operated with acurrent larger than a continuous rating range, so that a large weldingcurrent can be supplied to workpieces.

Still another object of the present invention is to provide a DCresistance welding apparatus having means for individually detectingfailures of parallel inverters, such as output reductions orovercurrents thereof, so that all the inverters can quickly beinactivated in response to detection of such a failure.

Yet another object of the present invention is to provide a DCresistance welding apparatus in which the trailing edges of switchingsignals are equalized in timing depending on the distorting action ofwelding transformers connected to parallel inverters, thereby preventingovercurrents from being generated at the trailing edges of inverteroutputs for increased efficiency.

Yet still another object of the present invention is to provide a DCresistance welding apparatus in which a signal corresponding to anenergized condition of workpieces to be welded can indirectly andaccurately be produced from a change in a current upon switchingoperation of inverters, and the application of electric energy to theworkpieces is stopped based on the signal thus produced, so that theworkpieces can appropriately be welded.

A further object of the present invention is to provide a DC resistancewelding apparatus comprising a pair of welding electrodes forsandwiching workpieces to be welded therebetween, and a circuitconnected to the welding electrodes for supplying a welding current tothe welding electrodes, the circuit comprising a plurality ofparallel-connected rectifying circuits each comprising a weldingtransformer and rectifying means.

A still further object of the present invention is to provide the DCresistance welding apparatus wherein the welding transformer includes asecondary winding having a central tap, and the rectifying meanscomprises a pair of rectifiers, the central tap serving as one outputterminal of each of the rectifying circuits, the secondary windinghaving one end connected to one terminal of one of the rectifiers, andthe other end connected to one terminal of the other of the rectifiers,the other terminals of the rectifiers being connected to each other asanother output terminal of each of the rectifying circuits, the oneoutput terminals of the rectifying circuits being connected together toone of the welding electrodes, and the other output terminals of therectifying circuits being connected together to the other of the weldingelectrodes.

A yet further object of the present invention is to provide theresistance welding apparatus further including an inverter unitcomprising a plurality of inverters connected respectively to therectifying circuits for driving the rectifying circuits.

A yet still further object of the present invention is to provide the DCresistance welding apparatus further including main control meansconnected to the inverters, and current detecting means for generating asignal proportional to the welding current supplied from the circuit tothe welding electrodes and feeding the signal back to the main controlmeans, the main control means having feedback control means for keepingthe welding current constant based on the signal fed from the currentdetecting means to the main control means.

Another object of the present invention is to provide the resistancewelding apparatus further including auxiliary control means connectedrespectively to the inverters, and current detecting means forgenerating signals proportional to branch welding currents suppliedrespectively from the rectifying circuits and feeding the signals backto the auxiliary control means, the auxiliary control means havingfeedback control means for keeping the branch welding currents constantbased on the signals fed from the current detecting means to theauxiliary control means.

Still another object of the present invention is to provide the DCresistance welding apparatus further including auxiliary control meansconnected respectively to the inverters, first current detecting meansfor generating signals proportional to branch welding currents suppliedrespectively from the rectifying circuits and feeding the signals backto the auxiliary control means, the auxiliary control means havingfeedback control means for keeping the branch welding currents constantbased on the signals fed from the first current detecting means to theauxiliary control means, main control means connected to the inverters,and second current detecting means for generating a signal proportionalto the welding current supplied from the circuit to the weldingelectrodes and feeding the signal back to the main control means, themain control means having feedback control means for keeping the weldingcurrent constant based on the signal fed from the second currentdetecting means to the main control means.

Yet another object of the present invention is to provide the DCresistance welding apparatus further including auxiliary control meansconnected respectively to the inverters, current detecting means forgenerating signals proportional to branch welding currents suppliedrespectively from the rectifying circuit and feeding the signals back tothe auxiliary control means, and main control means connected to theauxiliary control means for calculating command values indicative ofbranch welding currents to be produced by the respective inverters basedon a command value for the welding current, and for supplying thecalculated command values to the auxiliary control means, the auxiliarycontrol means having feedback control means for keeping the branchwelding currents constant based on the calculated command values, themain control means having timer means for synchronizing the branchwelding currents in timing.

It is also an object of the present invention to provide a DC resistancewelding apparatus a pair of welding electrodes for sandwichingworkpieces to be welded therebetween, an inverter unit havingsemiconductors switchable into and out of operation for converting DCelectric energy to AC electric energy, means for converting the ACelectric energy to DC electric energy and applying the DC electricenergy to the welding electrodes, a base driver for operating thesemiconductors with switching currents higher than a continuous ratingrange, and a timer circuit for measuring a conduction time of thesemiconductors and cutting off the switching currents when the measuredconduction time exceeds a predetermined time.

Another object of the present invention is to provide the DC resistancewelding apparatus wherein the time circuit comprises an oscillator forgenerating a pulsed signal having a frequency higher than the frequencyof the AC electric energy, an AND gate having an input terminal suppliedwith the pulsed signal and an input terminal supplied with a signalcorresponding to the AC electric energy, a presettable counter forcounting a pulsed signal from the AND gate and supplying a count signalto the base driver, and a setting unit for setting the presettablecounter to a preset value corresponding to the predetermined time.

Still another object of the present invention is to provide a DCresistance welding apparatus comprising a pair of welding electrodes forsandwiching workpieces to be welded therebetween, a plurality of powersupplies each comprising a converter, an inverter, a weldingtransformer, and a rectifier, for supplying a welding current producedfrom the rectifiers to the welding electrodes, detecting means fordetecting currents flowing either between the converters and theinverters or from the inverters of the power supplies, and for producingsignals indicating the detected currents, comparing means for comparingthe signals from the detecting means with predetermined levels and forproducing decision signals, and control means responsive to the decisionsignals for applying drive signals to the inverters to inactivate theinverters.

Yet another object of the present invention is to provide the DCresistance welding apparatus wherein the detecting means includeisolator means for removing noise from the signals produced by thedetecting means.

Yet still another object of the present invention is to provide a DCresistance welding apparatus comprising a pair of welding electrodes forsandwiching workpieces to be welded therebetween, a plurality of powersupplies each comprising a converter, an inverter, a weldingtransformer, and a rectifier, for supplying a welding current producedfrom the rectifiers to the welding electrodes, detecting means forproducing detected signals indicative of operation of the inverterssupplied with switching drive signals, means for comparing the detectedsignals and the switching drive signals and for producing distortingaction signals representing distorting actions of the weldingtransformers, and control means responsive to the distorting actionsignals for controlling leading edges of the respective switching drivesignals to equalize trailing edges thereof in timing.

A further object of the present invention is to provide the DCresistance welding apparatus wherein the control means comprises a clocksignal generator for generating a clock signal, a sawtooth generator forgenerating a sawtooth signal from the clock signal, and a comparator forcomparing the distorting action signals with the sawtooth signal toproduce switching drive signals having pulse durations determined by thedistorting action signals as a threshold level and trailing pulse edgesequalized in timing.

A still further object of the present invention is to provide a DCresistance welding apparatus comprising a welding gun for sandwichingworkpieces to be welded therebetween, a power supply unit comprising aconverter, an inverter, a welding transformer, and a rectifier, forsupplying a welding current produced from the rectifier to the weldinggun, detecting means for producing a detected signal corresponding to anoutput waveform from the inverter, leading/trailing edge currentdetecting means for producing current signals at leading and trailingedges of the detected signal, control means for applying a controlsignal to the leading/trailing edge current detecting means, andcalculating means for producing a differential current signal indicativeof a change in the resistance of the workpieces when the welding currentflows therethrough, from the current signals at leading and trailingedges of the detected signal, and applying the differential currentsignal to the control means.

A yet further object of the present invention is to provide the DCresistance welding apparatus further including de-energizing means forcutting off the welding current supplied to the welding gu when thedifferential current signal applied from the calculating means to thecontrol means is of a predetermined value.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which preferredembodiments of the present invention are shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram, partly in block form, of an electriccircuit of a conventional DC resistance welding apparatus;

FIG. 2 is a schematic view of a welding robot system incorporating a DCresistance welding apparatus according to an embodiment of the presentinvention;

FIG. 3 is a circuit diagram, partly in block form, of an electriccircuit of the DC resistance welding apparatus shown in FIG. 2;

FIG. 4 is a flowchart of an operation sequence of the DC resistancewelding apparatus;

FIGS. 5 through 7 are circuit diagrams, partly in block form, ofelectric circuits of DC resistance welding apparatus according to otherembodiments of the present invention;

FIG. 8 is a circuit diagram, partly in block form, of an electriccircuit of a DC resistance welding apparatus according to still anotherembodiment of the present invention;

FIGS. 9(a)-9(d) are diagrams illustrating operation of the DC resistancewelding apparatus shown in FIG. 8;

FIGS. 10 and 11 are circuit diagrams, partly in block form, of electriccircuits of DC resistance welding apparatus according to yet otherembodiments of the present invention;

FIG. 12 is a circuit diagram, partly in block form, of an electriccircuit of a DC resistance welding apparatus according to anotherembodiment of the present invention;

FIG. 13 is a block diagram of an abnormal current detector in the DCresistance welding apparatus shown in FIG. 12;

FIG. 14 is a diagram showing the manner in which direct currents arederived in the DC resistance welding apparatus shown in FIG. 12;

FIG. 15 is flowchart of a program of a system controller in the DCresistance welding apparatus illustrated in FIG. 12;

FIG. 16 is a block diagram of a DC resistance welding apparatusaccording to a further embodiment of the present invention;

FIG. 17 is a block diagram of a base drive pulse generator in the DCresistance welding apparatus shown in FIG. 16;

FIGS. 18(a)-18(d) are diagrams showing signal waveforms and timing,illustrative of operation of the DC resistance welding apparatus shownin FIGS. 16 and 17;

FIG. 19 is a block diagram of a DC resistance welding apparatusaccording to a still further embodiment of the present invention; and

FIGS. 20(a)-20(f) are diagrams showing signal waveforms and timing,illustrative of operation of the DC resistance welding apparatus shownin FIG. 19.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Like or corresponding parts are denoted by like or correspondingreference characters throughout the figures.

FIG. 2 schematically shows a welding robot system incorporating a DCresistance welding apparatus according to the present invention. Thewelding robot system basically comprises a welding robot 20, a robotcontroller 21, and a welding controller 22. The welding robot 20includes a first arm 26 mounted on a base 24 for rotation in thedirections indicated by the arrow, and a second arm 30 having one endpivotally coupled to the upper end of the first arm 26, the second arm30 being vertically movable in the directions indicated by the arrow bya cylinder 27 supported on the first arm 26. The cylinder 27 maycomprise a hydraulic cylinder or the like.

A gun assembly 34 is mounted on the other end of the second arm 30 by arotatable shaft 32. The gun assembly 34 comprises a welding transformerunit 36, a bracket 38 attached to the welding transformer unit 36, acylinder 40 fixed to the upper surface of the bracket 38, and a fixedgun arm 44 and a movable gun arm 46 which are supported on the bracket38 by a pivot shaft 42 mounted substantially centrally on the bracket38. Electrodes 48a, 48b are mounted on distal ends of the fixed andmovable gun arms 44, 46, respectively. The gun assembly 34 is angularlymovable by the rotatable shaft 32 in the directions indicated by thearrow. The movable gun arm 46 is movable toward and away from the fixedgun arm 44 in the directions indicated by the arrowheads A, B by acylinder 40 which is actuatable under a pneumatic pressure or the like.The first arm 26, the cylinder 27, the second arm 30, and the rotatableshaft 32 are electrically connected to the robot controller 21 by acable 41. The cylinder 40 and the welding transformer unit 36 areconnected to the welding controller 22 by a cable 43. The weldingcontroller 22 is electrically connected to a three-phase AC power supply64 by a cable 45 and also to the robot controller 21 by a cable 47. Therobot controller 21 controls the attitude of the welding robot 20 basedon teaching data, and gives a welding start command to the weldingcontroller 22. The welding controller 22 controls various weldingconditions such as the level and energization time of a welding currentwhich is supplied from the electrodes 48a, 48b to workpieces Wa, Wb.

FIG. 3 shows an electric circuit for controlling welding conditions ofthe DC resistance welding apparatus.

The electric circuit shown in FIG. 3 generally comprises the weldingcontroller 22 and the welding transformer unit 36. The weldingcontroller 22 includes a power converter assembly 50 and a control unit52. The power converter assembly 50 comprises a converter unit 56 havingfour converters 54a through 54d for converting three-phase AC electricenergy to DC electric energy, and an inverter unit 60 having fourinverters 58a through 58d for converting the DC electric energy to ahigh-frequency AC pulse train. The control unit 52 is basically composedof a main welding timer 53 serving as a main control means and auxiliarywelding timers 55a through 55d serving as auxiliary control means.

The converters 54a through 54d comprise rectifying diode stacks 62athrough 62d with their input terminals supplied with commercial ACelectric energy having a voltage of 400 V from the three-phase AC powersupply 64. The commercial AC electric energy is rectified by the diodestacks 62a through 62d, and smoothed by DC reactors 64a through 64d andcapacitors 66a through 66d of the converters 54a through 54d. DCvoltages V1 through V4 rectified and smoothed by the converters 54athrough 54d are then applied to the input terminals of the inverters 58athrough 58d. Each of the inverters 58a through 58d comprises a fullbridge of transistors.

The inverters 58a through 58d have respective output terminals connectedto primary windings 70a through 70d of four welding transformers 68athrough 68d of the welding transformer unit 36 which also have secondarywindings 72a through 72d, respectively. Transformer cores 73a through73d and an electrostatic shield electrode 75 are interposed between theprimary windings 70a through 70d and the secondary windings 72a through72d. The secondary windings 72a through 72d have output terminalsconnected to ends, such as anodes, for example, of pairs of rectifyingdiodes 74a through 74h whose other ends, i.e., cathodes, are connectedto each other in the respective pairs associated with the respectivewelding transformers 68a through 68d. The junctions at which thecathodes of the pairs of the diodes 74a through 74h are connected areconnected to current detectors 75a through 75d, respectively, such ascurrent transformers or Hall-effect devices for monitoring branchcurrents I1 through I4. The current detectors 75a through 75d are thenconnected in common to the welding electrode 48a through the fixed gunarm 44.

The secondary windings 72a through 72d have respective central taps 78athrough 78d which are connected in common to the other welding electrode48b through a current detector 80 such as a current transformer or aHall-effect device for monitoring a welding current I0 in the secondarywindings and also through the movable gun arm 46. The workpieces Wa, Wbare positioned and held between the electrodes 48a, 48b. The weldingcurrent I0 is therefore supplied to the welding electrodes 48a, 48b fromthe circuit composed of parallel-connected single-phase rectifyingcircuits with central taps.

The welding transformer unit 36 is composed of the four weldingtransformers 68a through 68d in view of electric specifications such ascurrent capacities of the rectifying diodes 74a through 74h which supplythe welding current, electric specifications such as current capacitiesof the full-bridge-connected power transistors of the inverters 58athrough 58d, and mechanical specifications such as weights of the gunassembly 34 supported by the second arm 30 of the welding robot 20. Theseparate welding transformers are more advantageous than a singlewelding transformer in that the semiconductors such as transistors anddiodes are smaller in size, the magnetic paths extending between theprimary and second windings across the transformer cores are shorter,and the transformers themselves are smaller and lighter as thetransformer cores have increased heat-radiating surface areas.

An output signal from the current detector 80, i.e., a signalproportional to the welding current I0, is fed to the main welding timer(main control means) 53 which monitors the welding current I0 and feedsback the same for control. The main welding timer 53 comprises aone-chip microprocessor having a CPU, a ROM, a RAM, and an interface,and serves as a system controller for effecting sequence control on awelding process. To the main welding timer 53, there are connected acondition setting unit 82 which sets welding conditions such as awelding current, an energization time, etc., and also an interface 84coupled to the robot controller 21. The main welding timer 53 and therobot controller 21 cooperate with each other in carrying out aninterlock operation.

Output signals from the current detectors 76a through 76d are applied tothe inverters 58a through 58d, respectively, through the auxiliarywelding timers 55a through 55d which serve as auxiliary welding controlmeans and have timer means and base drivers for energizing the bases ofthe full-bridge-connected transistors of the inverters 58a through 58d.The auxiliary welding timers 55a through 55d are connected to the mainwelding timer 53.

Each of the auxiliary welding timers 55a through 55d comprises aone-chip microprocessor having a CPU, a ROM, a RAM, and an interface,and have pulse-width-modulation (PWM) circuits with a fixed frequency.The auxiliary welding timers 55a through 55d controls the welding systemunder the control of the main welding timer 53.

The DC resistance welding apparatus of the present invention isbasically constructed as described above. Operation and advantages ofthe DC resistance welding apparatus will now be described with referenceto the flowchart of FIG. 4 which represents an algorithm of a programstored in the ROMs of the main welding timer 53 and the auxiliarywelding timers 55a through 55d and also in a ROM of the robot controller22.

The cylinder 40 (FIG. 2) is actuated by the welding controller 22 tomove the movable gun arm 46 in the direction indicated by the arrow A,thereby holding the electrodes 48a, 48b apart from each other. Then, thefirst arm 26 and the cylinder 27 are operated to vertically move thesecond arm 30 and rotate the rotatable shaft 32 based on teaching datastored in the robot controller 21, until the fixed gun arm 44 and themovable gun arm 46 are moved to a position to grip the workpieces Wa,Wb, in a step STP1.

Then, a welding start command is applied from the robot controller 21 tothe main welding timer 53 through the interface 84 of the weldingcontroller 22. Prescribed welding commands which are representative of awelding current and an energization time depending on the thickness andmaterial of the workpieces Wa, Wb are applied from the condition settingunit 82 to the main welding timer 5 in response to the welding startcommand in a step STP2.

Based on the applied welding commands, the main welding timer 53 readsdata corresponding to the welding current I0 for the workpieces Wa, Wbfrom its own RAM, and calculates current settings i1 through i4corresponding to branch welding currents I1 through I4 to be produced bythe respective inverters 58a through 58d, each of the current settingsil through i4 being 1/4 of the welding current I0, in a step STP3.

Thereafter, the cylinder 40 is actuated to move the movable gun arm 46in the direction indicated by the arrow B to bring the electrodes 48a,48b into abutment against the workpieces Wa, Wb, respectively. Theworkpieces Wa, Wb start being initially pressurized under an initialpressure read from the memory, in a step STP4.

Then, the main welding timer 53 reads data corresponding to theenergization time t0 from the RAM, and applies the welding currentsettings il through i4 and an energization start command to theauxiliary welding timers 55a through 55d in a step STP5.

The auxiliary welding timers 55a through 55d now controls energizationof the welding transformer unit 36 in a step STP7. More specifically,the auxiliary welding timers 55a through 55d simultaneously energize theinverters 58a through 58d, respectively, based on the welding currentsettings i1 through i4. Stated otherwise, the inverters 58a through 58dstart being energized synchronously by the respective auxiliary weldingtimers 55a through 55d.

High-frequency alternating currents having a frequency of 800 Hz, forexample, from the inverters 58a through 58d are transferred from theprimary windings 70a through 70d to the secondary windings 72a through72d of the welding transformers 68a through 68d, and then rectified bythe pairs of the diodes 74a through 74d connected to the outputterminals of the secondary windings 72a through 72d. Then, the rectifiedcurrents are added together and supplied as the welding current I0between the electrodes 48a, 48b. The auxiliary welding timers 55athrough 55d, which have feedback control means such ascomparator/amplifiers, compare signals corresponding to the branchcurrents I1 through I4 from the current detectors 76a through 76d withthe welding current settings i1 through i4 from the main welding timer53, and pulse-width-modulated currents to be applied to the bases of thetransistors of the inverters 58a through 58d for feedback control of thebranch currents I1 through I4. Therefore, the branch currents I1 throughI4 are equalized to the respective welding current settings i1 throughi4 and supplied as constant currents combined together to the workpiecesWa, Wb through the respective current detectors 76a through 76d.

The main welding timer 53 is supplied with a signal corresponding to thewelding current I0 from the current detector 80 which monitors thewelding current I0 passing between the workpieces Wa, Wb. If an abnormalcurrent is detected by the current detector 80, the main welding timer53 applies an energization end command to the auxiliary welding timers55a through 55d, and also supplies a welding error signal as aninterlock signal to the robot controller 21 through the interface 84.More specifically, if the welding current I0 falls within apredetermined range corresponding to a command value for the weldingcurrent I0 in a step STP8, then a step STP9 determines whether apredetermined energization time t0 has been reached or not. If theenergization time t0 has been reached, then the main welding timer 53applies an energization end command to the auxiliary welding timers 55athrough 55d in a step STP10. Then, the auxiliary welding timers 55athrough 55d simultaneously stop supplying the base currents to thetransistors of the inverters 58a through 58d in a step STP11.Accordingly, the inverters 58a through 58d driven by the respectiveauxiliary welding timers 55a through 55d are de-energized synchronouslyby the main welding timer 53. The supply of the welding current I0 tothe workpieces Wa, Wb is now cut off.

The workpieces Wa, Wb remain gripped and held between the fixed andmovable gun arms 44, 46 for a certain period of time while the weldingcurrent I0 is being cut off in a step STP12. During this time, a nugget(not shown) formed between the workpieces Wa, Wb is substantially fullysolidified, thus joining the workpieces Wa, Wb together. Subsequently, awelding process end signal is applied from the main welding timer 53 tothe gun assembly 34, whereupon the movable gun arm 46 is moved in thedirection indicated by the arrow A away from the fixed gun arm 44 by thecylinder 40. The workpieces Wa, Wb are retracted away from the gun arms44, 46, thus finishing the welding process in a step STP13.

With the aforesaid embodiment (hereinafter also referred to as a firstembodiment), since the inverters 58a through 58b are connected parallelto each other and the welding transformers 68a through 68d are alsoconnected parallel to each other, the DC gun assembly 34 may be small insize and light in weight. Therefore, the welding robot 20 with the gunassembly 34 attached to the second arm 30 thereof will not be reduced indurability. The gun assembly 36 can move in an increased space range asthe workpieces Wa, Wb and the gun assembly 36 of a reduced size are lessliable to physically interfere with each other.

Inasmuch as the load on the second arm 30 is reduced, the arms 26, 30 ofthe welding robot 20 can move at an increased speed, and hence the timeof each welding cycle is shortened. Since each of the transistors of theinverters 58a through 58d has a reduced current capacity, it is notnecessary to connect these transistors parallel to each other. This isadvantageous in that the transistors are not required to be sorted outfor well-balanced current capacities which would otherwise be taken intoconsideration if they were to be connected parallel to each other.Accordingly, the inverter unit 60 is highly stable in operation. If anyone of the inverters 58a through 58d fails, it can easily be replacedwith a new one. Thus, the inverter unit 60 can quickly be recovered froma malfunction caused by a failure of any one of the inverters 58athrough 58d.

In the above first embodiment of the present invention the auxiliarywelding timers 55a through 55d feed back the branch currents I1 throughI4 for making the welding current constant. However, currents to be fedback may not be detected from the output sides of the weldingtransformers 68a through 68d, but may be detected somewhere else. Forexample, the current detectors 76a through 76d may be connected to theinput terminals of the inverters 58a through 58d as shown in FIG. 5according to a second embodiment, or may be connected to the outputterminals of the inverters 58a through 58d as shown in FIG. 6 accordingto a third embodiment. The second and third embodiments shown in FIGS. 5and 6, respectively, are advantageous from the manufacturing standpointin that the current detectors 76a through 76d can be wired within thewelding controller 22 rather than the welding robot 22.

In the first through third embodiments above, the auxiliary weldingtimers 55a through 55d have feedback control means such ascomparator/amplifiers, and the main welding timer 53 monitors thewelding current I0 for an abnormal condition, but does not effectfeedback control. Consequently, the sum of the branch currents I1through I4 may not necessarily be equal in magnitude to the weldingcurrent I0 due to stray capacitances present in the gun arms 44, 46. Ifmore accurate control of the welding current I0 is required, a feedbackcontrol means such as a comparator/amplifier may be added to the mainwelding timer 53 shown in FIGS. 3 through 5 in accordance with a fourthembodiment, so that the welding current I0 can more accurately becontrolled under feedback control for agreement with a command currentvalue.

According to a fifth embodiment of the present invention, as shown inFIG. 7, no feedback control is effected by the auxiliary welding timers55a through 55d, but a feedback control means is added to only the mainwelding timer 53 for the control of the welding current I0. With thisarrangement, currents flowing through the transistors of the inverters58a through 58d are not equalized to each other and current flowingthrough the diodes 74a through 74h are also not equalized to each other.However, these currents may be equalized by selecting those transistorsand diodes which have either specifications and characteristics withsufficient margins or equal specifications and characteristics.

According to a sixth embodiment, the components of the inverters 58athrough 58d and those of the welding transformer unit 36 are sorted outfor close performance and specifications or performance andspecifications with sufficient margins, as with the fifth embodimentabove, and the feedback control means composed of the current detectors76a through 76d, 80, the main welding timer 53, and the auxiliarywelding timers 55a through 55d are dispensed with. In the sixthembodiment, no feedback control for the welding current I0 is carriedout but the welding current I0 is supplied with only a feed-forwardcommand.

In the first through sixth embodiments, the welding current to besupplied to the welding electrodes is produced by the current supplycircuit which is constructed of parallel-connected rectifying circuitseach comprising a welding transformer and rectifiers. This arrangementmakes it possible to employ a relatively large welding current to be fedto the workpieces, and also allows the welding gun assembly to be smalland light, thereby rendering the DC resistance welding apparatus wellaccessible for maintenance.

A DC resistance welding apparatus according to a seventh embodiment ofthe present invention will hereinafter be described with reference toFIGS. 8 and 9. According to the seventh embodiment, the time duringwhich the semiconductors such as transistors of the inverters areconductive is monitored to allow the semiconductors to be used withcurrents higher than a continuous rating current range, so that a largewelding current can be supplied to the workpieces. The seventhembodiment is not limited to the parallel connection of inverters andwelding transformers.

As shown in FIG. 8, the DC resistance welding apparatus basicallycomprises a converter unit 122 for converting three-phase DC electricenergy supplied from a three-phase AC power supply 120 to DC electricenergy, an inverter unit 124 for converting the DC electric energy fromthe converter unit 122, an output transformer 128 as a weldingtransformer for changing the voltage from the inverter unit 124 to adifferent voltage, a base driver 130 for supplying pulse-width-modulatedbase currents to full-bridge-connected semiconductors such as powertransistors 136a through 136d of the inverter unit 124, a controlcircuit 132, and a timer circuit 140. The base currents supplied fromthe base driver 130 are of a value large enough to make collectorcurrents of the transistors 136a through 136d higher than a continuousDC rating range.

The timer circuit 140 comprises an AND gate 141, an oscillator 142 forgenerating a pulsed signal having a frequency higher than that ofhigh-frequency AC electric energy produced from the inverter unit 124, acounter 144, and a setting unit 146 for setting the counter 144 to apreset count. The control circuit 13 supplies the timer circuit 140 witha timing gate signal Tm synchronous with a conduction time of thetransistors 136a through 136d driven by base driver 130. In response tothe timing gate signal Tm thus supplied, the counter 144 is operated tocount the energization time. If the counted conduction time becomeslonger than the preset count established by the setting unit 146, thenthe timer circuit 140 applies an output signal Toff to the base driver130, which then cuts off the base currents supplied to the transistors136a through 136d.

The converter unit 122 comprises a rectifying diode stack 134a, areactor 134b, and a capacitor 134c. The output transformer 12B comprisesa primary winding 152 a transformer core 154, and a secondary winding156 having a central tap 157. The secondary winding 156 is connected atits opposite ends to terminals of the rectifiers 131a, 131b whose otherterminals are connected in common to a welding electrode 158a. Thecentral tap 157 of the secondary winding 157 is connected to a weldingelectrode 158b. Workpieces Wa, Wb are held between the weldingelectrodes 158a, 158b.

Operation and advantages of the DC resistance welding apparatus of theseventh embodiment thus constructed will be described below.

The workpieces Wa, Wb are gripped between and initially pressurized bythe welding electrodes 158a, 158b. Three-phase AC electric energy fromthe three-phase AC power supply 120 is converted by the rectifying diodestack 134a, the reactor 134b, and the capacitor 134c of the converterunit 122 to DC electric energy which is applied to the inverter unit124.

The base driver 130 is supplied with a timing gate signal Tm (see FIG. 9at (a)) from the control circuit 132, and amplifies the timing gatesignal Tm into base currents large enough to energize the transistors136a through 136d connected in a full bridge of the inverter unit 124.The base currents are supplied to the respective transistors 136athrough 136d to make the transistors 136a through 136d conductive andnonconductive at successive times. The DC current supplied to theinverter unit 124 is now converted to AC electric energy.

The AC electric energy produced by the inverter unit 124 is thensupplied to the primary winding 152 of the output transistor 128, whichinduces AC electric energy of a different voltage across the secondarywinding 156. The AC electric energy induced across the secondary coil156 is rectified by the rectifiers 131a, 131b into DC electric energywhich is thereafter supplied through the welding electrodes 158a, 158bto the workpieces Wa, Wb.

As described above with respect to the prior art, the transistors of theinverter unit of the conventional DC resistance welding apparatus aredesigned such that they operate within a continuous rating range. If theoutput signals from a base driver are maintained at a levelcorresponding to a transistor conduction state and as a result thetransistors remain conductive, the transistors are prevented from beingbroken or destructed if they are within its thermal limits. Therefore,the actual level of electric energy applied to the inverter unit has tobe held within the continuous rating range. The conventional DCresistance welding apparatus has not been suitable for use inapplications wherein a large current is to be supplied to a weldingmachine.

The inventor has found that in a DC spot resistance welding machine, thetime (utilization factor) during which a current is passed between thewelding electrodes is considerably shorter than the time during whichthe DC spot resistance welding machine operates. For example, whenwelding workpieces of aluminum, the time during which a current issupplied to the electrodes is about 15 cycles (e.g., 0.3 sec.), and itsratio to the operation time of the welding machine including the time inwhich to energize the electrodes and move the workpieces is about 10%.If it is assumed that the inverter is switched at 800 Hz, then the timeduring which one of the transistors of the inverter is renderedconductive is 0.625 msec. at maximum, and remains nonconductive beyondthat period of time. It is known that semiconductors such as powertransistors, if used as switching elements of an inverter, can consume alarge current with respect to a continuous rating range. Therefore, thesemiconductors of the inverter in the spot resistance welding machinecan be used with a switching current higher than the continuous ratingrange. In this case, however, when the switching operation is stoppedand the semiconductors remain conductive, they tend to be broken. Theinverter therefore needs a protective circuit for protecting thesemiconductors from damage.

The timer circuit 140 is added for the purpose of protecting thetransistors 136a through 136d. The control circuit 132 supplies thetransistors 136a through 136d with current signals (base drive signals)through the base driver 130. When a timing gate signal Tm (see FIG. 9 at(a)), which is in synchronism with the conduction time of thetransistors 136a through 136d, is applied to the timer circuit 140, theAND gate 141 is enabled during a high level interval corresponding tothe conduction time, thus allowing output pulses S1 (see FIG. 9 at (b))from the oscillator 142 to pass through the AND gate 141. The pulsedsignal that has passed through the AND gate 141 is applied as a pulsedsignal S2 (see FIG. 9(c)) to the counter 144, which then starts to countthe pulsed signal S2. The counter 144 now measures the conduction timeof the transistors 136a through 136d. When the measured conduction timeexceeds a time ts preset in the setting unit 146 at a time tl, the timercircuit 140 produces an output signal Toff (see FIG. 9 at (d)). Theoutput signal Toff is applied to the base driver 130 to forcibly cut offthe base currents supplied from the base driver 130 to the transistors136a through 136d. Accordingly, in the event that the conduction time ofthe transistors 136a through 136d exceeds a predetermined time, thetransistors 136a through 136d are forcibly rendered nonconductive forprotection against damage.

The counter 144 starts counting pulses in response to a positive-goingedge of the timing gate signal Tm, and stops counting pulses and isreset in response to a negative-going edge of the timing gate signal Tmwhich is applied to a RESET terminal of the counter 144.

In the above embodiment, the time corresponding to the conduction stateof the transistors 136a through 136d is detected based on the base drivesignals (i.e., the timing gate signal Tm) from the base driver 130.According to an eighth embodiment of the present invention, however, theactual conduction time of the transistors 136a through 136d may bedetected as shown in FIG. 10. More specifically, a current detector 160comprising a Hall-effect device or the like is connected to an outputterminal of the inverter unit 124. An output signal from the currentdetector 160 is shaped in waveform by a waveform shaper 162 into such alevel that can enable the AND gate 141. The output signal from thecurrent detector 160 is applied to the AND gate 141 and the RESETterminal of the counter 144. With this arrangement, the timer circuit140 can monitor the actual conduction time of the transistors 136athrough 136d.

FIG. 11 shows a ninth embodiment in which two current detectors 160 areconnected to the input terminals of the inverter unit 124, and twowaveform shapers 162 are connected to the respective current detectors160. Output signals from the waveform shapers 162 are applied to thetimer circuit 140 through an electronic switch 164 such as a multiplierwhich is switched over each time a positive-going edge of the timinggate signal Tm is applied from the control circuit 132 to the electronicswitch 164. The timer circuit 140 can therefore measure the actualconduction time of the transistors 136a through 136d.

According to the seventh through ninth embodiments described above, theconduction time of the semiconductors of the inverter unit is monitoredby the timer circuit. When the semiconductors are conducted for a periodof time longer than a predetermined period of time, they are forciblyrendered nonconductive. Consequently, the semiconductors can be usedwithin a switching rating range, and can supply the welding robot with awelding current that is twice or three times larger than if they wereoperated within the conventional continuous rating range.

A DC resistance welding apparatus according to a tenth embodiment of thepresent invention will now be described below with reference to FIGS. 12through 15. According to the tenth embodiment, failures such as outputreductions or overcurrents of parallel inverters are individuallydetected, and all the inverters are quickly shut off in response todetection of such a failure.

As shown in FIG. 12, the DC resistance welding apparatus includes adevice 210 (including an ELB) for turning on and off three-phase ACelectric energy having a voltage of 400 V, and parallel-connected powersupplies A, B, C, D. The power supplies A, B, C, D respectively haveconverters 212a through 212d, inverters 214a through 214d, weldingtransformers 216a through 216d, and rectifiers 218a through 218d. DCelectric energy E1, which is the sum of output currents from therectifiers 218a through 218d, is supplied to a welding gun 220. Thewelding gun 220 has a pair of welding tips 220a, 220b for grippingworkpieces 222 therebetween.

Base drivers 226a through 226d are connected respectively to theinverters 214a through 214d for supplying base drive currents to thebases of switching transistors Tr1, Tr2, Tr3, Tr4 which are connected ina full bridge. The base drivers 226a through 226d operate on theprinciple of pulse width modulation for varying the on and off times ofpulses with a fixed frequency.

A system controller 230 comprises a one-chip microprocessor (weldingtimer) including a CPU, a RAM, a ROM, an I/O, and a PWM circuit. Thesystem controller 230 is connected to a setting means/central controlsystem including a computer for an FMS for effecting fully-closednumerical control, and carries out welding sequence control.

Three-phase AC electric energy of 400 V is rectified by the converters212a through 212d into DC electric energy, which is then converted backto high-frequency pulses (which will also be referred to as AC electricenergy) by the inverters 214a through 214d. This AC electric energy issupplied to the welding transformers 216a through 216d which convertsthe AC electric energy to large AC currents of a relatively low voltageof 10 V, for example. Then, the AC currents are rectified by therectifiers 218a through 218d by way of full-wave rectification intodirect currents which are added together into DC electric energy E1.

The system controller 230 is supplied with detected signals from failuredetectors (not shown), the signals indicating a bus fuse breakage, aprimary cable leakage, and temperatures of the switching transistors Tr1through Tr4 and the welding transformers 216a through 216d. In responseto these detected signals, the system controller 230 produces sequencecontrol signals for effecting certain controlling operations such asde-energization of the converters 212a through 212d and the inverters214a through 214d and shutting-off of the three-phase AC electric powerby operating the power supply on/off device 210.

The above arrangement is a known inverter-controlled power supply systemfor use with a welding robot.

Current detectors 232a through 232d and 234a through 234d each in theform of a toroidal coil are disposed around the wires interconnectingthe converters 212a through 212d and the inverters 214a through 214d ofthe power supplies A through D. These current detectors produce signalscorresponding to polarity-inverted currents Ia1, Ib1, Ic1, Id1 and Ia2,Ib2, Ic2, Id2 which flow upon switching operation of the inverters 214athrough 214d, and supply the produced signals to respective currentdetermining units 236a, 236b, 236c, 236d.

The current determining unit 236a will hereinafter be described by wayof example. The other current determining units 236b, 236c, 236d areidentical in construction to the current determining unit 236a and willnot be described in detail.

The current determining unit 236a has two signal processing sectionsconnected respectively to the current detectors 232a, 234a. The twosignal processing sections are structurally identical to each other.

As shown in FIG. 13, signals from the current detectors 232a, 234a aresupplied respectively to comparators 250, 252. The comparators 250, 252are also supplied with reference levels from variable resistors VR1,VR2, respectively, the reference levels being the same as the levels ofsignals generated by the current detectors 232a, 234a when the inverters214a through 214d operate normally. When the signals from the currentdetectors 232a, 234a are higher than the reference levels, as detectedby the comparators 250, 252, output signals are produced by thecomparators 250, 252. Noise is removed from the output signals from thecomparators 250, 252 by isolators 256, 258. The signals from theisolators 256, 258 are then supplied to latches 262, 264 in the form ofJK flip-flops which time the control operation of the system controller230 and also to AND gates 266, 268 of negative-true logic, from whichdecision signals S1a, S1b are produced.

It is assumed that the currents Ia1 through Id1 and Ia2 through Id2 withtheir polarity inverted on a time base are first constant as shown inFIG. 14 at (a) through (d), i.e., the power supplies A through D operatenormally. If the power supply A reduces or stops its power output aftera time t, then the power supplies B through D produce overcurrents afterthe time t (FIG. 14 at (f) through (h)). As a result, the currents Ib1through Id1 and Ib2 through Id2 are increased, and the currentdetermining units 236b through 236d apply decision signals S2a, S2bthrough S4a, S4b to the system controller 230. The system controller 230sequentially determines whether there are decision signals S1a, S1bthrough S4a, S4b or not. In this case, since the system controller 230first encounters a decision signal S2a, it is determined that theinverter 214b is producing an overcurrent. The system controller 230 nowstops supplying drive signals C3 (synchronous with a timing gate signal)through a signal distributor 232 in order to de-energize the switchingtransistors Tr1 through Tr4 of the inverters 214a through 214d, i.e., tointerrupt switching operation of the inverters. Therefore, in the eventof an output reduction or failure of the inverter 214a due to damage ofthe switching transistor Tr1, for example, because of a distortingaction of the welding transformer 216a, and the other inverters 214bthrough 214d produce overcurrents, all the inverters 214a through 214dare de-energized to prevent the other switching transistors from beingdamaged.

Any of the inverters 214a through 214d which has produced a reducedoutput or failed to produce an output may be visually indicated by anLED, for example, so that such a failing inverter, the inverter 214a inthe above example, can quickly be confirmed.

Sequence control of the system controller 230 for the control of thedrive signals C3 according to a program stored in the ROM of the systemcontroller 23 will be described below with reference to FIG. 15.

The program is executed by a welding start command from the settingmeans/central control system after the DC resistance welding apparatushas started operating.

First, the system controller 230 is instructed to receive decisionsignals S1a, S1b in a step 101. A next step 102 determines whether thesystem controller 230 has received decision signals S1a, S1b. If not,then control goes to a step 103, and if yes, control goes to a step 109.

Then, the system controller 230 is instructed to receive decisionsignals S2a, S2b in the step 103. A next step 104 determines whether thesystem controller 230 has received decision signals S2a, S2b. If not,then control goes to a step 105, and if yes, control goes to the step109.

The system controller 230 is instructed to receive decision signals S3a,S3b in the step 105. A next step 106 determines whether the systemcontroller 230 has received decision signals S3a, S3b. If not, thencontrol goes to a step 107, and if yes, control goes to the step 109.

The system controller 230 is instructed to receive decision signals S4a,S4b in the step 107. A next step 108 determines whether the systemcontroller 230 has received decision signals S4a, S4b. If not, then theprogram is ended, and if yes, control goes to the step 109.

In the step 109, the system controller 230 stops applying the drivesignals C3 to the base drivers 226a through 226d. Then, the program isbrought to an end.

After the step 109, the program starts being executed again. Theexecution of the program is repeated as long as the system controller230 is operated.

In the above embodiment, overcurrents from the converters 212a through212d are detected, and then the inverters 214a through 214d arede-energized. However, the current determining units 236a through 236dmay detect whether the inverters 214a through 214d have reduced orstopped their outputs, and stop operation of the inverters 214a through214d. Alternatively, the current detectors 232a through 232d and 234athrough 234d may be connected between the inverters 214a through 214dand the welding transformers 216a through 216d.

With the above embodiment, failures of the parallel inverters, i.e.,output reductions or overcurrents, are individually detected, and upondetection of such a failure, all the inverters are quickly de-energizedfor effective protection of the switching transistors of the inverters.

A DC resistance welding apparatus according to an eleventh embodiment ofthe present invention will be described with reference to FIGS. 16through 18.

As shown in FIG. 16, the DC resistance welding apparatus includes adevice 210 (including an ELB) for turning on and off three-phase ACelectric energy having a voltage of 400 V, and parallel-connected powersupplies A, B, C, D. The power supplies A, B, C, D respectively haveconverters 312a through 312d, inverters 314a through 314d, weldingtransformers 316a through 316d, and rectifiers 318a through 318d. DCelectric energy E1, which is the sum of output currents from therectifiers 318a through 318d is supplied to a welding gun 320. Thewelding gun 320 has a pair of welding tips 300a, 320b for grippingworkpieces 322 therebetween.

Base drivers 326a through 326d are connected respectively to theinverters 314a through 314d for supplying base drive currents to thebases of switching transistors Tr1, Tr2, Tr3, Tr4 which are connected ina full bridge. Detectors 328a through 328d in the form of toroidal coilsfor producing detected signals corresponding to switching waveforms fromthe converters 312a through 312d to the inverters 314a through 314d aredisposed around the wires interconnecting the converters 312a through312d and the inverters 314a through 314d. The detected signals from thedetectors 328a through 328d are supplied to respective base drive pulsegenerators 330a through 330d which generate and apply drive pulses S5,S6, S7, S8 to the base drivers 326a through 326d.

A system controller 340 comprises a one-chip microprocessor (weldingtimer) including a CPU, a RAM, a ROM, and an I/O. The system controller340 is supplied with control signals Cm from a setting means/centralcontrol system including a computer for an FMS for effectingfully-closed numerical control, and carries out welding sequencecontrol. The system controller 340 generates a setting signal Cr whichselects a switching frequency for the inverters 314a through 314d to seta change in the DC electric energy E1 applied to the welding gun 320,i.e., welding energy, to a desired level. The system controller 340applies the setting signal Cr to the base drive pulse generators 330athrough 330d.

A clock signal Cm is generated and applied by a clock signal generator342 to the base drive pulse generators 330a through 330d in response toan operation start command from the system controller 340.

The base drive pulse generators 330a through 330d are shown in detail inFIG. 17. They are identical in construction to each other, and thosereference characters placed in parentheses correspond to the base drivepulse generators 330b, 330c, 330d.

The base drive pulse generators 330a (330b, 330c, 330d) have integrators350a (350b, 350c, 350d) supplied with detected signals S1 (S2, S3, S4),and comparators 352a (352b, 352c, 352d) which are supplied with thesetting signal Cr and integrated signals from the integrators 350a(350b, 350c, 350d) and produce differential signals S10a (S10b, S10c,S10d). The base drive pulse generators 330a (330b, 330c, 330d) also havesawtooth generators 354a (354b, 354c, 354d) supplied with a clock signalCk from the clock signal generator 342 for generating sawtooth signalsS12a (S12b, S12c, S12d), and comparators 356a (356b, 356c, 356d)supplied with the differential signals S10a (S10b, S10c, S10d) and thesawtooth signals S12a (S12b, S12c, S12d). The comparators 356a (356b,356c, 356d) produce drive pulses S5, S6, S7, S8 with the differentialsignals S10a (S10 b, S10c, S10d) used as a threshold level.

The circuit arrangement composed of the clock signal generator 342 andthe base drive pulse generators 330a through 330d operate on theprinciple of pulse width modulation for varying the on and off times ofpulses with a fixed frequency.

Operation of the DC resistance welding apparatus shown in FIG. 12 willbe described below.

When control signals Cm are supplied to the system controller 340, aclock signal Ck is supplied to the sawtooth generators 354a through354d. Sawtooth signals S12a through S12d generated by the sawtoothgenerators 354a through 354d and differential signals S10a through S10dcorresponding to detected signals S1 through S4 are applied to thecomparators 356a through 356d. Drive pulses S5 through S8 with thedifferential signals S10a through S10d used as a threshold level aresupplied to the base drivers 326a through 326d. The drive pulses S5through S8 are divided and shaped in waveform by the base drivers 326athrough 326b, and then supplied to the bases of the switchingtransistors Tr1 through Tr4 of each of the inverters 314a through 314dto turn on the transistors Tr1 through Tr4. DC electric energy suppliedfrom the converters 312a through 312d to the inverters 314a through 314dis now converted to pulsed high-frequency AC electric energy. This ACelectric energy is then supplied to the welding transformers 316athrough 316d by which it is converted to large currents having arelatively low voltage of 10 V, for example. The currents from thewelding transformers 316a through 316d are rectified by the rectifiers318 a through 318d by way of full-wave rectification into directcurrents which are combined into DC electric energy E1.

The above arrangement is a known inverter-controlled power supply systemfor use with a welding robot.

If any one of the welding transformers 316a through 316d causes adistorting action due to a change in heir impedance resulting from afluctuation in the load presented by the workpieces, then the pulseduration of one of the detected signals S1 through S4 is varied.Therefore, a corresponding one of the differential signals S10a throughS10d varies as shown in FIG. 18. It is assumed here that the detectedsignal S2 varies as indicated at (b) in FIG. 18.

From the sawtooth signals S12a through S12d supplied to the comparators356a through 356d are generated drive pulses S5 through S8 using thedifferential signals S10a through S10d. The generated drive pulses S5through S8 are then applied to the base drivers 326a through 326d.

As can be seen from FIG. 18, the drive pulses S5 through S8 havetrailing edges equalized to each other in timing, so that no peakycurrents or overcurrents are generated at the trailing edges of theoutput waveforms from the inverters 314a through 314d.

While the detectors 328a through 328d are connected between theconverters 312a through 312d and the inverters 314a through 314d in theillustrated embodiment, the detectors 328a through 328d may be connectedbetween the inverters 314a through 314d and the welding transformers316a through 316d.

In the embodiment shown in FIGS. 16 through 18, the trailing edges ofbase drive pulses are equalized in timing to cope with a distortingaction of the welding transformers associated with the plural powersupplies. Since no overcurrents are generated at the trailing edges ofthe output signals from the inverters, the DC resistance weldingapparatus has increased efficiency.

FIGS. 19 and 20 illustrate a DC resistance welding apparatus accordingto a twelfth embodiment of the present invention. According to thisembodiment, a signal corresponding to an energized condition ofworkpieces is indirectly and accurately produced from a change in acurrent upon switching operation of inverters, and the application ofelectric energy to the workpieces is stopped based on the signal thusproduced, so that the workpieces can appropriately be welded. Thisembodiment is not limited to parallel-connected inverters andparallel-connected transformers.

As shown in FIG. 19, the DC resistance welding apparatus comprises aninverter-controlled power supply unit A including a welding gun, adetector unit B (detecting means) for producing a signal correspondingto an output signal (waveform) upon switching operation of the powersupply unit A, a differential signal calculating unit C (correspondingto a leading/trailing edge current detecting means and a calculatingmeans) for generating leading and trailing edge current signals from thesignal produced by the detector unit B, and generating and applying adifferential current signal, and a control unit D (control means) forgenerating a control signal for the detection of leading and trailingedge currents) and effecting the closed-loop control of the entireapparatus.

The power supply unit A has a converter 412, an inverter 414, a weldingtransformer 416, and rectifiers 418a, 418b. DC electric energy E1combined by the output terminals of the rectifiers 418a, 418b is appliedto a welding gun 420. The welding gun 420 has a pair of welding tips420a, 420b for gripping workpieces 422 therebetween.

The detector unit B comprises a current detector in the form of atoroidal coil or the like, which is disposed around the wireinterconnecting the inverter 414 and the welding transformer 416 forproducing a detected signal S1a corresponding to switching operationsignal S1.

The differential signal calculating unit C comprises track holdamplifiers 424, 426 which are supplied with timing signals St1, St2 forholding the leading and trailing edges of the detected signal S1a, and alogarithmic amplifier 428 which is supplied with output signals S3, S4from the track hold amplifiers 424, 426 through respective resistors R1,R2 and which produces an analog logarithmic signal S6.

The control unit D comprises a welding system controller, for example,and includes a base driver 430 for applying drive signals to the basesof switching transistors Tr1, Tr2, Tr3, Tr4 connected in a full bridge,a timing generator 432 for producing timing signals St1, St2, and a PWMcircuit 434 for producing a signal processed by the principle of pulsewidth modulation which varies the on and off times of pulses with afixed frequency, and applying the produced signal to the base driver430. The control unit D also has a microprocessor (MPU) 436 including aCPU, a RAM, a ROM, and an I/O. The microprocessor 436 controls the DCresistance welding apparatus under closed-loop control, and is suppliedwith the calculated signal S6 and a welding start command signal Cc froma setting means/central control system including a computer for an FMSfor effecting fully-closed numerical control, and carries out weldingsequence control.

To the control unit D, there are connected a monitor 440 for visuallydisplaying numerical values and waveforms representative of dataprocessed by the various components of the apparatus, and a setting unit442 for indicating numerical values and waveforms to be displayed andestablishing control instructions in cooperation with the MPU 436.

The DC resistance welding apparatus of the twelfth embodiment willoperate a follows:

In response to a welding start command signal Cc applied to the MPU 436,the MPU 436 operates the PWM circuit 434. Drive signals from the basedriver 430 are applied to the inverter 414 to start operating theinverter 414. Three-phase AC electric energy of 400 V is rectified bythe converter 412 to DC electric energy, which is converted by theinverter 414 to a pulsed high-frequency AC switching operation signalS1. This signal S1 is then supplied to the welding transformer 416 whichconverts the signal S1 to a large current having a relatively lowvoltage of 10 V, for example. The alternating current is rectified bythe rectifiers 418a, 418b by way of full-wave rectification into DCelectric energy E1.

The detector unit B produces a detected signal S1a (FIG. 20 at (a))corresponding to the switching operation signal S1 produced by theinverter 414. The detected signal S1a is then supplied to the track holdamplifiers 424, 426, which are also supplied with timing signals St1,St2 (FIG. 20 at (b) and (c)) that correspond to a leading edge (currentI) and a trailing edge (current I0) of the detected signal S1a. Thetiming signals St1, St2 are generated by the timing generator 432 basedon the signal from the PWM circuit 434. The track hold amplifiers 424,426 produce output signals S3, S4 and apply them through the respectiveresistors R1, R2 to the logarithmic amplifier 428. The output signalsS3, S4 have levels V1, V2, respectively, corresponding to the currentsI, I0 at the leading and trailing edges, as shown in FIG. 20 at (d) and(e). The logarithmic amplifier 428 generates an analog logarithmicsignal S6 through an analog logarithmic process defined by: ##EQU1##where L is the inductance of the secondary winding of the weldingtransformer 416, R is the resistance of the secondary winding of thewelding transformer 416, T is the time during which the detected signalS1a remains low in level, and o is a constant.

The signal S6 thus produced is of a value based on the assumption thatthe resistances of the rectifiers 418a, 418b and the welding gun 420 areconstant and can be ignored. An area indicated by m in FIG. 20 at (a)corresponds to the amount of energy consumed by the workpieces 422 whenthe welding current passes therethrough, i.e., a change in theresistance of the workpieces 42 due the formation of a nugget at thetime the workpieces 422 are welded together.

The signal S6 is supplied through a squarer (not shown) to the MPU 436which converts the signal to a digital signal by quantization. Then, thedigital signal is sampled with a timing signal shown in FIG. 20 at (f),and the numerical value and waveform of the sampled signal are visuallydisplayed by the monitor 440.

In this manner, a change in the resistance of the workpieces 422 grippedby the welding tips 420a, 420b at the time the welding current passesthrough the workpieces 422 can indirectly be obtained from the currentsI, I0 at the leading and trailing edges of the switching operationsignal S1 of the inverter 414.

By referring to the displayed numerical value and waveform, the operatorcan apply electric energy to workpieces 422 of a different kind whilecontrolling the DC power E1 applied between the welding tips 420a, 420bso as to achieve a high mechanical strength without expulsion andsurface flash which would otherwise be experienced empirically andexperimentally.

The control of the DC power E1 is effected as follows: Times forstarting and stopping energization and a conduction time are enteredthrough the setting unit 442 and the monitor 440 into the RAM in the MPU436.

Then, the workpieces 422 are fed and fixed according to variousfully-closed numerical control modes on the production line controlledby setting means/central control system including a computer for an FMS.After the workpieces 422 have been pressurized by the welding gun 420,the MPU 436 instructs the PWM circuit 434 to apply a signal to the basedriver 430 based on the times for starting and stopping energization andthe energization time which are stored in the RAM thereof. In thismanner, the workpieces 422 can be welded for a high mechanical strengthwithout expulsion and surface flash.

With the embodiment shown in FIGS. 19 and 20, a signal corresponding tothe energized condition of the workpieces is indirectly obtained basedon a change in the current from the inverter. Consequently, any wire fordetecting a change in the voltage between the welding tips orelectrodes, i.e., a change in the resistance of the workpieces upongrowth of a nugget through which the welding current passes, is notrequired. Any noise produced when the detected voltage is dropped or thecurrent is passed is reduced, and the change in the resistance of theworkpieces at the time the welding current passes therethrough can beproduced highly accurately.

Furthermore, based on the resistance of different workpieces at the timethe welding current passes through, the times for starting and stoppingenergization of workpieces, i.e., the supply of the welding current tothe workpieces, and the energization time can appropriately becontrolled.

Although certain preferred embodiments have been shown and described, itshould be understood that many changes and modifications may be madetherein without departing from the scope of the appended claims.

What is claimed is:
 1. A DC resistance welding apparatus comprising:apair of welding electrodes for sandwiching workpieces to be weldedtherebetween; a circuit connected to said welding electrodes forsupplying a welding current to said welding electrodes, said circuitincluding a plurality of parallel-connected rectifying circuits eachhaving a welding transformer and a rectifier; a plurality of invertersconnected respectively to said rectifying circuits for driving saidrectifying circuits; auxiliary control means, connected respectively tosaid inverters, for energizing said inverters; current detecting meansfor generating signals proportional to branch welding currents suppliedrespectively from said rectifying circuits and for feeding said signalsback to said auxiliary control means; and main control means, connectedto said auxiliary control means, for calculating command valuesindicative of branch welding currents to be reproduced by the respectiveinverters based on a command value for the welding current and forsupplying the calculated command values to said auxiliary control means;said auxiliary control means including feedback control means formaintaining the branch welding current constant based on said calculatedcommand values; said main control means including timer means forsynchronizing production of said branch welding currents.
 2. The DCresistance welding apparatus according to claim 1, wherein said weldingtransformer includes a secondary winding having a central tap and saidrectifying circuit includes a pair of rectifiers, said central tapserving as one output terminal of each of said rectifying circuits, saidsecondary winding having one end connected to one terminal of one ofsaid rectifiers and the other end connected to one terminal of the otherof said rectifiers, the other terminals of said rectifiers beingconnected to each other as another output terminal of each of saidrectifying circuits, said one output terminals of said rectifyingcircuits being connected together to one of said welding electrodes, andthe other output terminals of said rectifying circuits being connectedtogether to the other of said welding electrodes.
 3. The DC resistancewelding apparatus according to claim 1, further comprising:secondcurrent detecting means for generating a signal proportional to thewelding current supplied to said welding electrodes and for feeding saidsignal back to said main control means; said main control means havingfeedback control means for maintaining said welding current constantbased on said signal fed from said second current detecting means tosaid main control means.
 4. The DC resistance welding apparatusaccording to claim 1, wherein said feedback control means maintains saidbranch welding currents constant based on said signals fed from saidcurrent detecting means to said auxiliary control means.
 5. The DCresistance welding apparatus according to claim 1, further comprising:aplurality of converters, connected respectively to said inverters, forsupplying a DC current to said inverters; said current detecting meansdetecting currents flowing either between said converters and saidinverters or currents from said inverters; said current detecting meansproducing signals indicating the detected currents; and comparing meansfor comparing the signals from said detecting means with predeterminedlevels for producing decision signals; said main control means beingresponsive to said decision signals to inactivate the inverters.
 6. TheDC resistance welding apparatus according to claim 5, wherein saiddetecting means includes isolator means for removing noise from thesignals produced by said detecting means.
 7. A DC resistance weldingapparatus comprising:a pair of welding electrodes for sandwichingworkpieces to be welded therebetween; a plurality of power supplies,each including a converter, an inverter, a welding transformer, and arectifier, for supplying a welding current produced from the rectifiersto said welding electrodes; detecting means for producing detectedsignals indicative of operation of the inverters supplied with switchingdrive signals; means for comparing said detected signals and saidswitching drive signals and for producing distorting action signalsrepresenting distorting actions of said welding transformers; andcontrol means, responsive to said distorting action signals, forcontrolling leading edges of the respective switching drive signals toequalize trailing edges thereof.
 8. The DC resistance welding apparatusaccording to claim 7, wherein said control means includes a clock signalgenerator for generating a clock signal, a sawtooth generator forgenerating a sawtooth signal from said clock signal, and a comparatorfor comparing said distorting action signals with said sawtooth signalto produce switching drive signals having pulse durations determined bythe distorting action signals as a threshold level and trailing pulseedges.